Study: Cannibals Usually Dine Alone

Cannibalism
is rampant in the animal kingdom, including among some humans in the past. Since germs can sneak from victims to predators,
one might suspect diseases linked to cannibalism would prove widespread.

Instead,
diseases spread by cannibalism are rare. New calculations suggest this is
because cannibals usually dine alone. If cannibals do feast together, germs
could begin taking advantage.

"Maybe
this is why cannibalism is no longer common among people as it was in the past,
because of the strong negative effects it can have when transmitting a
disease," ecologist Volker Rudolf at the University of Virginia at
Charlottesville told LiveScience.

Cannibalism
is "stunningly common in nature," Rudolf explained. "In the past, there were
misconceptions that cannibalism was something that only happened in labs or
under extreme conditions. But it isn't."

Even though
people now typically abhor cannibalism, "it was quite a common practice in
many human societies in the past," Rudolf added.

Scientists
do know of diseases transmitted solely or mostly through cannibalism. For
instance, Kuru is a degenerative brain ailment in Papua New Guinea, akin to mad cow
disease, that only afflicts people who eat human flesh.

Since
cannibalism is widespread,
Rudolf and his colleague Janis Antonovics wanted to see how commonplace
diseases spread by cannibalism were. Scientific archives showed that while
cannibalism was seen in animals ranging from crustaceans and insects to mammals, it
apparently was only the predominant route for transmission of disease in two
cases--Kuru in humans, and the protozoan Sarcocystis in lizards, in which
cases the reptiles snack on
each others tails.

To investigate the prevalence question, the researchers fused calculations used to forecast how diseases spread
throughout a population with ones used to estimate how predator and prey levels rise and fall in relationship
to one another in the context of cannibalism.

After
calculating how diseases jump from predator to prey in cannibalism, the
researchers discovered that cannibalism was only an effective means of spread
if cannibals dined together rather than solo on victims, findings detailed in the May 7 issue of
the Proceedings of the Royal Society B.

"Such
group cannibalism is really rare in nature," Rudolf said. "Besides
humans, the only other example we could find are with chimpanzees,
where males group together to attack and consume other chimps. Cannibalism is
very common in nature, but it's usually one-on-one."

In cases
where group cannibalism does occur in nature, however, it might prove fruitful
to investigate, the researchers noted. For instance, the young of certain
insects and spiders are known to devour their own mothers, while male lions
are known to eat cubs after they acquire new harems. Rudolf hopes to
investigate how cannibalism and group dynamics affect each other with
experiments in insects.

"This
is exciting work," said population ecologist Nat Holland at Rice University
in Houston. It raises the question of whether or not epidemics of sexually
transmitted diseases in humans are related to the size of groups in orgies.
"Those are social diseases as well," he said.

Charles Q. Choi

Charles Q. Choi is a contributing writer for Live Science and Space.com. He covers all things human origins and astronomy as well as physics, animals and general science topics. Charles has a Master of Arts degree from the University of Missouri-Columbia, School of Journalism and a Bachelor of Arts degree from the University of South Florida. Charles has visited every continent on Earth, drinking rancid yak butter tea in Lhasa, snorkeling with sea lions in the Galapagos and even climbing an iceberg in Antarctica.